The present invention relates generally to joining graphite to graphite, and particularly to bonding or resistance brazing a plurality of graphite members together with a thermally sensitive material.
Graphite is used in many industrial fields, including chemical, electrical, metallurgical, electrochemical, nuclear, and rocket fields. In several of these areas of manufacture, it is desirable to join graphite to graphite. In the field of electrochemistry, graphite is widely used as an electrode material due to its electrical and thermal characteristics, and because it is one of the most inert materials with respect to chemical reactions. In this particular application, it is important to achieve a low transition or contact resistance between the graphite members being joined in order to minimize voltaic losses.
One such electrochemical application is the zinc-chloride battery, where graphite is employed for both the positive and negative electrodes. During the charging of the battery, zinc metal is electrodeposited on the negative or zinc electrode and chlorine gas is generated at the positive or chlorine electrode from an aqueous zinc-chloride electrolyte. During the discharging of the battery, the reactions are reversed to generate electricity from the terminals of the battery. The zinc electrode is constructed from dense or fine grained graphite, and the chlorine electrode is constructed from a liquid permeable porous graphite.
Three techniques generally have been employed to join graphite electrodes or members together. Bolt connections have been used, but this approach increases the design complexity and results in an unnecessary loss of graphite material. A second technique has been set forth in U.S. Pat. No. 3,813,301 issued May 28, 1974, entitled "Process Of Charging And Discharging A Metal Halogen Cell", and is herein incorporated by reference. In this patent, a bipolar electrode structure is described wherein a porous graphite member is cemented to a dense graphite member with conducting "carbonizable" cement. The cement may be composed of a number of materials, including phenol formaldehyde, which upon heating will at least partially be converted to carbon. The electrodes are cemented together by applying the cementing material at the contact place between the electrodes, and heating the electrodes sufficiently to carbonize the cement. Preferably the material resulting from this heat treating should contain as much carbon as possible to afford a good electrical contact. Accordingly, it is taught to mix the cementing material with carbon or graphite.
The third technique of joining graphite to graphite is described in U.S. Pat. No. 4,100,332, issued on July 11, 1978, entitled "Comb Type Bipolar Electrode Elements And Battery Stacks Thereof", and is herein incorporated by reference. In this patent, it is taught to provide for a press or interference fit between the graphite electrodes and the graphite bus-bar wall. Accordingly, the electrodes are constructed slightly thicker than the grooves in the bus-bar, so that when pressed in they may be retained by a pressure fit. It is also stated that the electrodes may be connected to the bus-bar by cementing, plasma spraying at the point of contact, or welding.
The present invention provides a novel method of joining graphite to graphite which results in a low transition or contact resistance. Particularly, the method comprises: interposing a thermally sensitive material between the graphite members to be joined, applying pressure forcing the graphite members together, applying sufficient heat to the graphite members to melt the thermally sensitive material, and providing a period of time for cooling before releasing the applied pressure. The thermally sensitive material may be composed of any suitable plastic, metal, or ceramic material generally having a low melting point or glass transition temperature. Preferably, the thermally sensitive material is composed of a thermoplastic material, such as polyvinyl chloride, in the form of a powder. The heating may be provided by any suitable means, preferably resistance brazing. An important feature of the present invention is that the thermally sensitive material need not be heated to the point where it undergoes destruction or carbonization. It need only be heated to the point where the material enters the liquid phase so that it may distribute throughout the joint being formed and the pores of the graphite surfaces by capillary action. Since graphite is thermally quite stable (it sublimes at approximately 6600.degree. F.), a wide variety of materials may be used for the thermally sensitive bonding material. However, thermoplastic materials are preferred due to their generally low melting points and low cost. It has been found that when thermoplastic material is resistance brazed to graphite, an acceptably low transition or contact resistance on the order of 0.5 milli-ohms/cm.sup.2 may be achieved.
Other features and advantages of the present invention will become apparent in view of the drawings and the following detailed description of the preferred embodiments.